Automatic volume control circuits



Jan. 2i, w36. H. A. SNOW AUTOMATIC VOLUME CONTROL CIRCUITS 5 Sheets-.sheet 1 Filed Oct. 18, 1932 .Fam 2i, 1936. H. A. SNOW 2,028,359

AUTOMATIC VOLUME CONTROL CIRCUITS Filed Oct. 18,` 1932 5 Sheets-Sheet 2 INVENTOR- ATTORNEY- im 21, 1936. MSNOWW 2,028,35

,AUTOMATIC VOLUME CONTROL CIRCUITS Patented Jan. 21, 1936 UNITED STATES AUTOMATIC VOLUME CONTROL CIRCUITS Harold A. Snow, Mountain Lakes, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 18, 1932, Serial No.v638,323 12 Claims. v(Cl.(25020) .My present invention relates to amplifier gain A control circuits and more particularly to anautomatic control arrangement adapted for use in connection with amplifiers.

`One of the main objects of my present invention is to provide an improved type of automatic gain control system for low or high frequency ampliers, which system is of the general type utilizing a linear electron discharge control tube. 10 Since a system of automatic amplifier gain control should only function when the signal voltage applied to the detector reaches a certain desired value, and then operate to prevent this detector voltage increasing above this value, the characl teristic of such an automatic control should be non-linear. Since it is desirable to make the characteristic of the detector o1' a receiver as nearly linear as possible, it is therefore preferable to employ a non-linear electron discharge tube 20 device between the detector and the amplifier to be controlled. In general, such a combination of a non-linear electron discharge control tube, furnishing grid bias to a radio frequency amplier for controlling the gain thereof, and a detector is known. However, in the present application there are disclosed various improvements in such known arrangements, and these improvements comprise, as stated heretofore, one of the main objects of this invention. 30 Another important object of the present invention is to provide a method of controllingrthe radio gain in a radio receiver by utilizing a direct current voltage furnished by the detector, and amplifying this voltage with a direct current amplier which may be non-linear, thus permitting the use of a linear detector and a non-linear direct current amplifier in an automatic gain control system for a radio receiver, such a system being highly desirable in view ofthe fact that the 40 eiiciency of the direct current amplifier makes the gain control system highly sensitive.

Another important object of the present invention is to provide a detector arrangement in a radio receiver wherein the drop of potential 5 across an impedance path in thefdetector cathode is utilized for obtaining a radio frequency bias control Voltage, a direct current amplifier being arranged to amplify the drop of potential across said cathode impedance path, and the amplified 50 potential drop being utilized asthe radio frequency amplifier gain control voltage.

Another object of the present invention is to provide an automatic gain control system employing a detector arranged for cooperation with a 55 direct Vcurrent amplier adapted to amplify the direct current component of the detector output, the amplifier and detector circuits being arranged for energization from a single unit voltage supply, the direct current amplifier being of the screen grid type and having an impedance in 5 its anode circuit connected tothe radio frequency grids to be controlled in such a manner that the grid of the first radio frequency stage is biased more than the grid of one or more succeeding stages.

Still another object of the present invention is to provide an automatic volume control system fora radio receiver wherein the direct current component of thedetector output is amplified in a direct current amplifier of the screen grid type, the screen electrode of the amplifier being connected to a point in the detector cathode circuit, and one or more input electrodes of the radio frequency stages being connected to an impedance in the anode circuit of the direct curren amplifier.

And still other objects of the invention are to improve Vgenerally the simplicity and eiiciency of automatic gain control devices, and to particularly provide a radio frequency gain control system of the type using a direct current amplifier in Yconjunction with the detector output which is not onlyeconomical, durable'and reliable in operation, but economically manufactured and assembled. V

The novel features which I believe to be char.- acteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to theV following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings,

Fig. 1 diagrammaticallyf shows a receiver embodying the present invention,

Fig. 2 shows a modified form of the invention, and

Figs. 3 to 6 inclusive each'show different modications o1 the invention.

Referring now to the accompanying drawings wherein like reference characters designate similar circuit elements in the different figures, the invention is first, described in connection with Fig. 2, in order to point out the general principles underlying the present invention. VIn this figure is shown in diagrammatic manner an arrangement for automatically controlling the gain of one or more stages of radio frequency amplification, the gure showing only the detector tube I, and the automatic volume control tube (A. V. C.) 2 in order to preserve simplicity of description.

The control voltage for the radio frequency grids is provided by the resistor R, the lead 3 connected to the negative side of the resistor R being connected to the grids of the radio frequency tubes. The control tube 2 is of the indirectly heated cathode type, and has its cathode connected by a lead 4, terminating in a slider 5, to a point of about +40 volts on the voltage supply potentiometer connected between terminals +B and -B. The anode of tube 2 is connected through the resistor R to the grounded conductor 6 which is connected to a point on the voltage supply potentiometer of about volts.

The anode of the detector tube I is connected to the audio frequency system in any manner Well known to those skilled in the art, and the cathode of the detector tube is connected by means of a lead 1, including a grid bias resistor R1 in series therein, to the negative terminal -B of the voltage supply potentiometer. The grid of tube 2 is connected by a lead 8 to a point a between the positive terminal of resistor R1 and thecathode of the detector tube. Between the detector grid and the negative side of the resistor R1 is connected a tunable input circuit 9 which includes a variable tuning condenser Ill, it being clearly understood that the circuit 9 is to be coupled to the output of a preceding ampliiier, Whether it be a tuned radio frequency amplifier of a plurality of stages, or the intermediate frequency amplifier of a superheterodyne receiver. The resistor R1 is shunted by an audio frequency by-pass condenser Il.

As pointed out heretofore, the present gain control system utilizes the voltage drop across a resistor in the cathode circuit of the detector for controlling the gain of the radio frequency stages, the voltage drop being amplified by means of a direct current amplifier prior to utilization in connection with the radio frequency stages. Thus, the drop of potential across the resistor R1 of the detector tube I is utilized for this purpose. This voltage drop increases approximately linearly With the signal volt-age impressed on the grid of tube I. The volume control tube 2 increases the current through the resistor R when the potential drop across the resistor R1 reaches a proper value, and the voltage drop across the resistor R is then impressed as a control voltage on the radio frequency amplifier grids.

The slider 5 serves to regulate the bias adjustment for the automatic volume control tube 2, and thus sets the level at which the radio frequency detector Voltage is held. No current flows through the control tube plate circuit when no radio frequency signal voltage is on the detector grid. When this voltage increases to a predetermined value, the direct current through the detector bias resistance R1 increases by a certain value which decreases the bias on the control tube, and permits current to flow through the resistor R, thus biasing the radio frequency amplifier tubes and holding the detector signal input voltage substantially constant.

While in Fig. 2 the lead 8 has been shown as iixedly connecte-d to point a in the cathode circuit of detector tube I, it is to be understood that only a portion of the drop across resistor R1 may be used to operate the control tube 2. Thus, it

is to be clearly understood that the point a may be, if desired, the lend oi' a slider adapted to be adjustably connected to the resistor R1. In that case the sensitivity of the control tube 2 could be readily adjusted. Also, the cut-off level could be adjusted by either the slider 5 or the point a, and either of these could then be used as a manual volume control if desired. This type of direct current amplifier gain control has the advantage that it is operated more nearly by actual carrier level and is less aiiected by modulation, this being true especially for' high carrier voltages on the detector, sincel the direct current bias .of the detector becomes more nearly linear in its variation with respect to the detector radio frequency carrier Voltage with high carrier voltages.

In Fig. l is shown a conventional type of radio receiver embodying the arrangement shown in Fig. 2, with certain modiiications. The receiver comprises the usual radio frequency amplifier tubes I2, I3, followed by a screen grid detector tube I4, the control tube I5 also being of the screen grid type. A voltage supply potentiometer is shown connected between terminals +B and +B, a point on the potentiometer, having a potential of about +50 Volts, being connected to ground through a conductor I9, the latter functioning as the ground line of the receiver. The anode of tube I2 is connected to a proper point of positive potential on the supply potentiometer through the primary coil of the coupling transformer M, and the screen grid electro-de is similarly connected to a point of less positive potential on the voltage supply potentiometer.

The cathode of tube I2, as Well as the cathode of tube I3, is grounded by connection to the lead I6, and includes a grid bias resistor R2. The Variable tuning condensers C in each of the input circuits of tubes I2, I3 and I4 have their rotors grounded by connection to lead I6. A conventional uni-control arrangement I'I, shown in dotted lines, is shown connected to the three tuning condensers, it being understood that any desired type Yof mechanical uni-control arrangement may be used for simultaneously operating these tuning condensers.

The control grid of tube l2 is connected to the ground lead I5 through a coil I8 in series with a resistor I9, the usual antenna circuit 20 being adjustably connected to the resistor I9 to provide a proper input point to the stage including tube I2. The anode and screen grid circuits of tube I3 are energized from the voltage supply potentiometer in a manner similar to that described in connection with tube I2. The coupling transformer M1 is utilized to couple the anode circuit of tube I3 to the tunable input circuit of detector tube I4. The control grid of tube I4 is coupled to the high potential side of its tunable input circuit through a signal coupling condenser.

The coupling condenser has a magnitude of about 0.0001 microfarad while the anode of the tube is connected to a point on the supply potentiometer having a potential of about +230 v. through a path which includes the choke coil 2 I, having a magnitude of about 10 mh., and the primary coil 22 of the audio frequency coupling transformer Mz. The secondary coil of the transformer M2 is arranged for connection to any well known type of push-pull power amplier stage.

Each side of the choke coil 2| is connected to the grounded lead l through a condenser 23, each condenser having a magnitude of about 0.0001 microiarad. The screen grid electrode of tube I4 is connected through a lead 24 to a point on the supply potentiometer having a potential of about+125 Volts. The cathode of detector tube I4 is connected through a path which includes the lead 25, the resistor R1 and the lead 20 to terminal -B of the supply potentiometer. The resistor R1 functions as the detector grid bias resistor, has a value of about 25,000 ohms, and is adapted to furnish a bias of -10 to -30 volts. The grid of detector tube Ill is connected through a resistor R3, having a magnitude of about 2 megohms, and lead 21 to the negative side of the resistor R1.

The potentials for the various electrodes of the control tube I are obtained by connecting a secondary potentiometer between point X on lead 24 and point X1 on lead 25. The screen grid electrode of tube I5 is connected to a point A of a potential of about +30 volts on the secondary potentiometer, the resistor 28 between points X and A having a magnitude of about 10,000 ohms; the resistor 29 between points A and Y having a magnitude of about 500 ohms; the resistor 30 between points Y and Z having a magnitude of about 1,500 ohms; while the resistor 3l between points Z and X1 has a magnitude of about 1,000 ohms.

The cathode of control tube I5 is connected for adjustable movement between points Z and Y on resistor 30, and functions as a manual volume control instrumentality for the receiver. The slider 32 is arranged in any well known manner to be adjusted between these last two mentioned points, it being pointed out that the point X1 u is at a potential of zero volts; the point Z is at a potential of volts; while the point Y is at a potential of volts. v

The anode of control tube I 5 is connected by a lead 33 to the negative terminal of the control bias resistor R which has a magnitude of about '75,000 ohms, the positive side of the resistor R being connected, through the ground lead I6 and resistor B, to the cathode of tube I5. The negative terminal of resistor R is connected by a lead 34 to the low potential side of coil I8. Thus the negative side of resistor R is connected tothe grid of radio amplifier tube I2, while the grid of tube I3 is connected through a lead 35 to a point on the resistor R which divides the resistor R into two portions. The upper portion b has a magnitude of about 50,000 ohms, while the lower portion e has a magnitude of about 25,000 ohms. It will, thus, be seen that the control bias on the grid of tube I2 is different from, and greater than, the control bias applied to the grid of amplifier tube I3. The function of this differential control biasing will be explained hereinafter. Y

The control electrode of tube I5 is connected to the positive side of the resistor R1. The potential drop across this latter resistor furnishes the grid bias voltage for the detector tube I 4, and also varies the grid bias voltage for the control tube I5. The tube I5 amplies the drop across the resistor R1 and the varying potential drop across resistor R, due to the variation in plate current of control tube I5, functions to automatically control the gain of the amplifier stages including tubes I2 and I3. Of course, the voltage supply potentiometer can be connected to a direct current supply source, or may be connected to the output of the usual filtered rectified alternating current source. The various control grid, cathode, screen grid and anode circuits in the radio frequency and detector stages have been shown appropriately by-passed for radio frequency currents by fixed condensers.

The operation of this type of automatic gain control system was found to be satisfactory. The secondary resistance network between points X andX1, whichY is shunted across the potentiometer portion B, is necessary to supply the automatic volume control tube I5. If the automatic Volume control voltages are obtained directly from portion B a reaction is caused by virtue of the Ip of the radio frequency tubes. This results in a reaction at certain settings of the manual control 32 producing a sudden increase in radio frequency bias. By the shunt network X, X1 this effect is eliminated since in' this case a reduction of radio frequency Ip increases the voltage across the network X, X1 as decreasing radio frequency Ip decreases the current and the IR drop across the portion C1 of the supply potentiometer. This system, is therefore, perfectly stable. That is, the phase of the reaction of the automatic volume control tube on the supply voltage is reversed over what it is when the automatic volume control tube is supplied, directly from the portion B, and these effects can be controlled by adjusting the positions of point X along the potentiometer portion B.

It is believed that the operation of the arrange- 1 ment shown in Fig. 1, as well as that shown n Fig. 2, will be clear from the aforegoing description and the drawings. V/hen the carrier input to the detector, which is of the power detector type, exceeds the desired level giving the desired reproduction volume, the potential drop across the resistor R1 increases with the result that the 'bias on the grid of control tube I5 decreases.

This results in a greater plate current flow through the plate circuit of tube I5, and a greater drop of potential across the resistor R.

Obviously, then, the negative voltages applied to the grids of tubes I2 and I3 increase, and the amplification in the stages including the tubes i2 and I3 decrease, thus bringing the reproduction volume down to the desired level. If the carrier decreases below the desired level, then the reverse action takes place, and the radio frequency amplication is automatically increased to'again provide the predetermined input voltage to the power detector tube I4. Thus, it will be seen that the function of the direct current aml plier tube I5, and its associated circuits, and differential biasing connections to the tubes I2 and I3 is to provide at all times a uniform input voltage supply to the input of power detector tube Id. Such a uniform supply is especially desirable in the case of a power detector which is to operate in a linear fashion.

Accordingly, it will be seen that there is provided by means of the present invention an automatic gain control system wherein a .linear power detector is employed for handling the signal energy, 'and a non-linear direct current amplifier for controlling automatically the gain of the radio frequency stages, itbeing observed that the drop across the resistor R1 varies substantially linearly with signal input voltages to the detector tube I4.

The purpose of the differential biasing arrangement of tubes I2 and I3 is clearly disclosed by `Stuart Ballantine in application Serial N o. 376,163 iled July 5, 1929. Briefly, it may be stated that such an arrangement extends the range of amplication control throughout which distortionless amplification may be obtained.

It has been found experimentally that if the several grids of the radio frequency stages are biased equally, a value of common grid bias is reached at which the relation between the input and the output voltages of the radio frequency amplifier becomes non-linear. In accordance With the sai-d Ballantine invention, the range of gain control of the radio frequency stages is graduated, and the range through which the bias Voltage 4of the second radio frequency stage may be varied is less than the bias voltage range of the first tube. By departing, in the manner shown in Fig. l, from the prior practice of employing a common control grid bias for all stages, the range of automatic distortionless control is greatly extended for the radio frequency stages.

In Figs. 3 to 6 inclusive are shown modified forms of the invention, the modifications particularly being made in the -direct current amplier portion of the amplifier gain c-ontrol system. Thus, in Fig. 3 the detector i4 is of the screen grid type, as in Fig. 1, and the circuit arrangement of Fig. 3 is schematically shown in the same manner as in the case of Fig. 2. The control tube 2 is also of the screen grid type, and only those portions of the circuit in Fig. 3 will be described which involve the modification. The cathode of the detector tube I4 is connected to the grid of control tube 2, and both the cathode of tube I4 and grid of tube 2 are connected through the detector grid bias resistor R1 to the -B terminal of the supply potentiometer.

The screen grid of control tube 2 is shown adjustably connected by a lead 45.9 to a point of +75 volts on the supply potentiometer. The anode of control tube 2 is shown connected through the control bias resistor R to the grounded lead 6. The cathode of tube 2 is connected by the slider 5, functioning as a manual volume control or setting device for the radio frequency detector voltage level, to a point in the region of +20 volts of the supply potentiometer.

The operation of the arrangement shown in Fig. 3 is similar to that described in connection with Figs. 1 and 2. The potential'drop across resistor R1 increases or decreases with increasing and decreasing inputs to the detector i4, and the potential drop across resistor R varies in the same manner as the potential drop across resistor R1. Thus, there is a device for controlling the input to the detector so that the input is substantially constant. It should be noted that the screen gri-d of tube 2 may be connected as shown, or connected to the anode thereof thus rendering it possible to use tube 2 as a triode. Of course, the adjustable lead 40 may be connected so that the screen is below plate voltage; that is, it has a normal screen grid operation. The present circuit is very sensitive, and it should also be noted that the grid of the automatic volume control tube may be shifted along the resistor R1 to set the level of detector voltage or to manually contr-ol volume, as well as by controlling the' position of slider 5.

Fig. 4 shows a modification where the screen grid of the control tube is used to control its plate current, the screen of control grid tube 2 being connected by a lead 2 to a point between the positive side of the resistor Ri and the cathode of the detector tube. The control grid of the control tube 2 is connected between the negative side of the resistor R1 and the -B terminal of the supply potentiometer.

The cathode of the control tube 2 is, again, connected to the supply potentiometer through a slider 5 which functions to manually control the detector input voltage level. The operation of this modification is similar to that described in connection with Fig. 3, it being noted that the plate current flow through the resistor Ris varied by the varying potential on the screen grid of tube 2. If the signal input to the detector increases, the potential on the screen grid of tube 2 increases, with the result that the plate current flow through the resistor R increases, and the radio frequency amplification is, thus, cut down. The arrangement shown in Fig. 4 reduces the necessary B supply voltage to about 30 volts below ground potential for automatic volume control operation.

In Fig. 5 is shown a modification of the arrangement shown in Fig. 4 for use with detectors having higher bias voltage. It will be noted that the control grid of control tube 2 is connected by a lead 5 to the -B terminal of the supply potentiometer, while the plate of the control tube 2 is operated below the screen grid potential to conserve B supply voltage. Otherwise, the operation of this modification is similar to that shown in Fig. Li.

Fig. 6 shows still another modified form of direct current amplifier arrangement, wherein the change in detector plate current is used to change the plate voltage of the direct current amplifier tube. type. Thus, the anode of the tube 2 is shown connected to a lead 6 which is connected between the positive side of the detector cathode resistor R1 and a point on the supply potentiometer.

The control grid of tube 2 is connected to the -B side of the potentiometer, while point d of the supply potentiometer is established as the ground potential by connecting it, through a lead 1', to the negative side of resistor R1 and grounding it at that side. The cathode of tube 2 is connected to the grounded negative side of resistor R1 through the control bias resistor R.

It should be noted that the voltage across the resistor R is in reverse direction for what is desired in normal bias volume control. The operation of this modification is based on the fact that the voltage across R1 in the plate-cathode circuit of the detector decreases with increasing signal inputs. The positive side of resistor R is, therefore, connected to the grids of the radio frequency amplifiers. Of course, the normal fixed bias at no-signal condition on each controlled amplier will be secured by keeping the amplifier cathode more positive than its grid by the desired bias value. As signal amplitude increases the voltage drop across R1 decreases;

this causing a decreased drop across R and a reduction'in the gain of the controlled amplifier by a less positive biasing of the controlled amplifier grid. It is to be noted that instead of connecting the anode of tube 2 to the lead 6 the tube 2 could be replaced by a screen grid tube, and the screen grid thereof connected to lead 5 in a manner as shown in connection with the anode of the tube 2.

While I have indicated and described several systems for carrying my invention into'eiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

The detector in this case is of the grid leak Y direct current amplifier arranged to amplify the direct current component of the detector output current developed across said detector cathode resistor, and connections between the amplifier stages and the direct current amplifier for applying the output of said direct current amplifier to the cascaded amplifier stages so that the voltage change applied to successive stages is progressively decreased,

2. The combination with a plurality of cascaded amplifier stages whose input circuits are Vmaintained at a common carrier wave potential, of means for varying an amplification controlling voltage applied to eachof said stages, said voltage variation means comprising a detector, an input circuit for said detector and shunted across the output of said amplifier stages, a direct current amplier arranged to amplify the direct current component of the detector output current, connections between the amplifier stages and the direct current amplifier for applying the output of said direct current amplifier to the cascaded 4amplifier stages so that the voltagechange ap-Y plied to successive stages isv progressively decreased, said detector including a screen grid tube, a resistor in the cathode circuit of the detector, the input electrodes of the direct current ampliiier being connected to the said resistor inthe cathode circuit of the detector, the detector grid being connected to a point on said resistor which` is negative with respect to the detector cathode, and the anode circuit of the direct currentV amplifier including a resistor, the potential drop across the latter being employed for said voltage change applied to the successive amplifier stages.

3. In combination, a radio frequency amplifier having an input circuit arranged to have impressed thereon signal energy to be amplified, a detector including an impedance in its output circuit whereby the detector space current flows through the impedance, a direct current amplifier having its input electrodes connected across said impedance, the detector grid being connected to a point on said impedance which is negative with respect to the detector cathode, an impedance in the output circuit of said direct current amplifier, connections between said first amplifier and said anode impedance for applying a control bias to the grid of said first named signal energy amplifier, and a common voltage supply potentiometer for energizing the electrodes of said signal energy amplifier, detector and direct current amplifier.

4. In combination, a radio frequency amplifier having an input circuit arranged to have impressed thereon signal energy to be amplified, a detector including an impedance in its output circuit whereby the detector space current flows through the impedance, a direct current amplifier having its input electrodes connected across said impedance, the detector grid being connected to a point on said impedance which is negative with respect to the detector cathode, an impedance in the output circuit of said direct current amplifier, connections between said rst amplifier and said anode impedance forvapplying a control bias to the grid of said'first named signal energy amplifier, a common voltage supply potentiometer for energizing the electrodes of said signal energy amplifier, detector and direct current amplifier, said direct current amplifier including an adjustable slider in its cathode circuit, the slider being arranged for adjustment over a portio'n of said potentiometer.

5. In combination, a radio frequency amplifier having an input circuit arranged to have impressed thereon signal energy to be amplified, a detector' including an impedance in its output circuit whereby the detector space current fiows 4through the impedance, a direct current amplifier having its input electro'desconnected across said impedance, the detector grid being connected to a point on said impedance which is negative with respect to the detector cathode, an impedance in the output circuit of said direct current amplifier, connections between said first amplifier and said anode impedance for applying a control bias to the grid of said first named signal energy amplifier, a common voltage supply potentiometer for energizing the electrodes of said signal energy amplifier, detector and direct current amplifier, said detector including a screen grid tube, and said direct current amplifier comprising a screen grid tube, the screen ygrid and cathode of the direct current amplier being adjustably connected to said voltage supply potentiometer.

6. In combinati-on, with a radio frequency amplifler arranged to have its gain automatically controlled, a detector, an impedance in the cathode circuit of the detector, a direct current amplifier comprising a screen grid tube, means for connecting the screen electrode of the direct current amplifier to one side of said impedance, and the grid an-d cathode to the other side of said impedance, an impedance in the anode circuit of the direct current amplifier, and means'for connecting the low potential side of said anode impedance to an input electrode of said amplifier to' be controlled.

7. In combination with a radio frequency amplier arranged to have its gain automatically controlled, a detector, an impedance in the cathode circuit of the detector, a direct current amplifier comprisinga screen grid tube, means for connecting the screen electrode of the direct current amplifier to one side of said impedance, andthe grid and cathode to the other si-de of said impedance, an impedance in the anode circuit of thedirect current amplifier, means for connecting the low potential side of said anode impedance to an input electrode of said amplif'ler to be controlled, and a common potential supply potentiometer connected to energize the circuits of said detector and direct current amplifier tubes.

8. In combination, in a radio receiver, a power detector stage including a screen grid tube, a multi-stage radio frequency amplifier arranged to supply a relatively high radio frequency signal voltage toi the input of said screen grid detector,` a common supply voltage potentiometer for energizing the circuits of said multi-stage amplier and power detector, means in the output circuit of said power ,detector arranged for connection to a power amplifier stage, an impedance in the cathode circuit of said rscreen grid detector tube, the detector grid being connected to a point on said impedance which is negative with respect to the detector cathode potential to insure linear operation of the detector, a direct current screen grid amplifier having its electrodes connected to said potentiometer for energizaticn therefrom, the control grid and cathode of sai-d direct current amplifier being connected across said impedance, an impedance in the anode circuit of the direct current amplifier, and connections between said anode circuit impedance and vthe `grids of said multi-stage amplifier for applying control biases to said grids which progressively decrease for each successive stage.

9. In combination, in a radio receiver, a power detector stage including a screen grid tube, a multi-stage radio frequency amplifier arranged to supply a relatively high radio frequency signal voltage to the input of said screen grid detector, a common supply voltage potentiometer for energizing the circuits of said multi-stage ampliiier and power detector, means in the output circuit of said power detector arranged for connection to a power amplifier stage', an impedance in the cathode circuit of said screen grid 'detector tube, the vdetector grid being connected t-o a point on said impedance which is negative with respect to the .detector cathode potential to insure linear operation of the detector, a direct current screen grid amplifier having its electrodes connected to said potentiometer for energization therefrom, the control gri-d and cathode of said direct current amplier being connected across said impedance, an impedance in the anode circuit of the direct current ampliiier, connections between said anode circuit impedance and the grids of said multi-stage amplifier for applying control biases to said grids which progressively decreases for each successive stage, and means for adjustably connecting the cathode of said direct current amplifier to said potentiometer.

l0. In combination, in a radio receiver, a power detector stage including a screen grid tube, a multi-stage radio frequency amplifier arranged to supply a relatively high radio frequency signal voltage to the input of said screen grid detector, a common supply voltage potentiometer for energizing the circuits of said multistage amplifier and power detector, means in the output circuit of said power detector arranged for connection to a power amplifier stage, an impedance in the cathode circuit of said screen grid detector tube, the detector grid loeing connected to a point on said impedance which is negative with respect to the detector cathode potential to insure linear operation of the detector, a direct current screen grid ampliiier having its electrodes connected to said potentiometer for energization therefrom, the control grid and cathode of said direct current ampliiier being connected across said impedance, an impedance in the anode circuit of the direct current ampliiier, connections between said anode circuit impedance and the grids of said multistage amplifier for applying control biases to said grids which progressively decrease for each successive stage, the connection between the electrodes of said direct current amplifier and said potentiometer including a secondary potentiometer in shunt with said rstV named potentiometer.

11. In combination, in a radio receiver, a power detector stage including a screen grid tube, a multi-stage radio frequency amplifier arranged to supply a relatively high radio frequency signal voltage to the input of said screen grid detector, a common supply voltage potentiometer for energizing the circuits of said multistage ampliiier and power detector, means in the output circuit of said power detector arranged for connection to a power amplifier stage, an impedance in the cathode circuit of said screen grid detector tube, the detector grid being connected to a point on said impedance which is negative with respect to the detector cathode potential to insure linear operation of the detector, a direct current screen grid amplier, having its electrodes connected to said potentiometer for energization therefrom, the control grid and cathode of said direct current amplifier being connected across said impedance, an impedance in the anode circuit of the direct current amplifier, connections between said anode circuit impedance and the grids of said multi-stage amplifier for applying control biases to said grids which progressively decreases for each successive stage, and manually operated means connected to one of the input electrodes of said direct current amplifier for setting the detector input voltage level.

l2. In a radio receiver of the type which includes a radio frequency amplifier and a detecto-r network provided with a tube having a resistor in its cathode circuit, the improvement which comprises a gain control tube having a cathode, anode and at leastone auxiliary cold electrode, said cathode and auxiliary electrode being connected to points of diierent potential on said resistor, an impedance in the anode circuit of said control tube, amplifier gain control connections between the amplifier and said impedance, said points on the resistor being chosen to cause the space current flow of said control tube to increase-with increase of signal amplitude, and the detector grid being connected to a point on the resistor which is negative with respect to the detector cathode potential whereby the detector operates as a biased detector.

HAROLD A. SNOW. 

